Biologically active peptides are generally first synthesized as inactive, higher molecular weight precursors. Processing of a proprotein precursor, by enzymatic cleavage and covalent modifications, yields active peptide(s) from the larger proprotein. It is not uncommon for a proprotein to be processed such that more than one biologically active peptide is produced from the same precursor molecule. For example, cholecystokinin, proopiomelanocortin, calcitonin, proglucagon, and proadrenomedulin each produce several different biologically active peptides.
Amidation is often a biologically important post-translational modification, as the amidated form of a protein generally is biologically active and more resistant to carboxypeptidases. An amidation motif has been used to identify potential cleavage/amidation sites in precursor proteins which may result in the generation of biologically active amidated peptides from a precursor protein (see, for example, Eberlein et al., J. Biol. Chem., 267:1517-1521, 1992; Siegfried et al., Proc. Natl. Acad. Sci. USA, 89:8107-8111, 1992; Quinn et al., Cancer Cells, 3:504-510, 1991; Cuttitta, The Anatomical Record, 236:87-93, 1993; Fenger and Johnsen, Biochem. J., 250:781-788, 1988; Orskov et al., J. Biol. Chem., 264:12826-12829, 1989). The amidation motif consists of an invariant glycine residue followed by a region of basic amino acids on the carboxy-terminal side of the glycine residue.
Although the free acid and amidated forms of a peptide are difficult to distinguish structurally, the amide can be 100-1000 times more biologically active than the free acid form of the peptide (Cuttitta, The Anatomical Record, 236:87-93, 1993). Amidated peptides can exhibit the same type of biological activity as other peptides processed from the same precursor protein, although their activity may vary with peptide size (Tatemoto et al., Biochem. Biophys. Res. Comm., 251:471-476, 1998).